1. Field of the Invention
This invention relates to a method and apparatus for digitally measuring the configuration of a lens frame of an eyeglass frame or the configuration of a template obtained by copying the lens frame, and more particularly to a frame configuration measuring method and apparatus suitable for use in combination with a lens grinding machine for grinding an optical lens blank in accordance with data concerning the configuration of the lens frame or template.
2. Description of the Prior Art
As shown in FIG. 20(a), in a conventional apparatus for measuring the configuration of a lens frame, right and left lens frames (frame rims) 3, 3' of an eyeglass frame 2 are first brought into contact with a surface 1 (measurement reference surface) of a frame holding device, not shown, and are maintained thereon by a maintaining rod, not shown, provided with a spring. Thereafter, as shown in FIG. 21(a), a beveled feeler 5 is brought into contact with a V-shaped groove 4 of the lens frame 3 and moved along the groove 4, so that the track of the feeler 5 is three-dimensionally detected to measure the configuration of the lens frame 3. Likewise, the configuration of the other lens frame 3' is measured. In FIG. 21(a), reference characters 4a, 4b each designate a slope of the V-shaped groove 4.
Generally, the front of the eyeglass frame 2 is curved as shown in FIGS. 20(a) and 21(a). For this reason, part 7 of the lens frame 3' close to a temple (sidepiece) of the eyeglass frame 2 is spaced from the measurement reference surface 1 when part of the lens frame 3' close to a bridge 6 between the lens frames 3, 3' is maintained in contact with the surface 1 by means of the maintaining rod.
In order to measure the accurate size and shape of the lens frames 3, 3', a coincidence is required between angle .gamma. and angle .delta.. As shown in FIG. 21(b), .gamma. is an angle formed by a plane including the apex of the edge of a ground lens L and a center line passing through the bottom of of the V-shaped groove 4, whereas .delta. is an angle formed by a plane including the apex of the feeler 5 and the center line passing through the bottom of of the V-shaped groove 4.
However, there is a case in which the edge of a lens blank (material lens) is ground according to incorrect data on the position of the apex of the feeler 5 obtained by the disagreement between angle .gamma. and angle .delta. because of the inclination of the lens frames 3, 3' with respect to the surface 1. In that case, the lens L is ground to have a size c different from a true size d thereof, as shown in FIG. 21(a). Consequently, as shown in FIG. 21(c), the finished lens L of the size c does not fit the lens frames 3, 3' of the size d.
To overcome this fault, the following method can be adopted. That is, the configuration of the lens frame 3 is first measured preliminarily, and then the inclination of the lens frame 3 with respect to the surface 1 is calculated from configuration data obtained by the preliminary measurement. Thereafter, in order to obtain true configuration data on the lens frame 3, a principal measurement is carried out such that the feeler 5 is controlled to fit the V-shaped groove 4 on the basis of the inclination of the lens frame 3 obtained above. Likewise, the configuration of the other lens frame 3' is measured. This measuring method is desirable for accurate measurement.
When the feeler 5 is moved along the groove 4, the feeler 5 is also moved in the direction of a Z-axis (direction perpendicular to the measurement reference surface 1) because of the inclination of the lens frames 3, 3' with respect to the surface 1. The minimum height hmin and maximum height hmax of the lens frame 3 in that direction are almost the same as those of the lens frame 3', respectively.
Accordingly, if the lens frames 3, 3' are shaped with almost the same accuracy, measurement data concerning one of the lens frames 3, 3' is available for the other lens frame.
However, in the case of a plastic eyeglass frame or an eyeglass frame having two lens frames which are shaped different from each other, configuration data obtained by mutual conversion often become different from those obtained by actual measurement. Therefore, there is need for the lens frames 3, 3' to be individually measured, but much time is consumed in the individual measurement.
Another problem is as follows. As shown in FIGS. 20(a) and 20(b), when the lens frames 3, 3' of the eye glass frame 2 are set on the surface 1, the lower parts 3a, 3a' of the lens frames 3, 3' close to the bridge 6 come into contact with the surface 1, so that the V-shaped grooves of the parts 3a, 3a' become parallel to the surface 1. Therefore, the feeler 5 is initialized to be first brought into contact with the V-shaped groove of the part 3a or 3a'when the measurement of a frame configuration is started.
The engagement of the feeler 5 with the V-shaped groove 4 is easily carried out in this manner if the lens frames 3, 3' of, for example, a metallic eyeglass frame 2 are almost uniform in thickness (thickness t in the up and down directions in FIG. 20(c)) and further the bottom of the V-shaped groove 4 is always positioned in the middle of the thickness t, as shown by the arrow A in FIG. 20(c).
However, in a plastic eyeglass frame, as shown by the arrow B in FIG. 20(c), the bottom of the groove 4 is often positioned apart from the middle of the thickness t and, in addition, the lens frames 3, 3' are not formed uniform in thickness.
In this case, the feeler 5 initialized to be inserted into the V-shaped groove 4 parallel to the surface 1 cannot be engaged with the groove 4 of the part 3a or 3a', hence measuring the configuration of the lens frames 3, 3' inaccurately. For this reason, heretofore, the feeler 5 has been manually engaged with the V-shaped groove 4 for the measurement.
In this connection, the following experimentation result was obtained. That is, even in the case of a plastic eyeglass frame or an eyeglass frame having two lens frames which are sized different from each other, in other words, even in the case of an eyeglass frame in which a relatively big difference is found between respective data obtained by preliminarily measuring the configurations of the right and left lens frames 3, 3', the minimum height hmin and maximum height hmax in the direction of the Z-axis of the feeler 5 in the right lens frame 3 are almost the same as those in the left lens frame 3', respectively, and further a difference between the heights hmin and hmax obtained by principal measurement is almost the same as that obtained by the preliminary measurement.